The long-term objective of this study is to understand the molecular mechanisms that underlie or direct endochondral bone development, using the formation of specific digit elements as a model for studying this process. Our lab has recently identified a brachydactyly mouse mutant (bdy[tg]) showing fore- and hind-limb digit loss. This mutation, generated by transgene insertion, has provided a means to identify, a new skeletogenic factor and to study its interaction with known bone regulators such as BMPs, Wnts, Ihh, PThrP; and Gli transcription factors. Given the conservation of signaling networks used for both endochondral bone formation and tooth development, what we learn by examining digit development should be important for understanding both skeletogenesis and development and maintenance of dento-alveolar structures. The bdy gene has been identified as the type IB bone morphogenetic protein receptor, BmprIB, and is required for formation of a digit cartilage template. Two major classes of transcripts arise from the BmprIB locus. Rather than representing alternative splice variants from a single promoter, the two isoforms are products from two BmprIB promoters: one located distally, driving expression in the developing limb skeleton, and one situated proximally, initiating transcription in neural epithelium. This proposal addresses the following questions: (1) What are the key cis-regulatory elements required for BmprIB expression in developing limb skeletal mesenchyme? (2) What are the transcription factors that bind these cis-elements? To address these questions, the following specific aims are proposed: (I) Delineate cis- regulatory elements required for expressing BmprIB in the limb. (II) Identify transcription factors that bind these required cis-elements.